Restore stand-alone exponential histogram mapping functions

[jmacd]

Problem Statement

The exponential histogram mapping functions in sdk/metric/aggregator/exponential/mapping have been reviewed and are meant for stand-alone use by other OpenTelemetry components as well as for future use by the OTel metrics SDK. They were left in an orphaned directory. The directory where this lands is not critically important.

Proposed Solution

Potentially the package named sdk/metric/metricdata/histogram/exponential makes sense, with subdirectories named mapping (the already reviewed part) and structure (the same is pending review into the orphaned directory in #3022).

Alternatives

Since the data structure and mapping functions are meant to be stand-alone, they do not need to be rooted in the sdk/metric package. They could go into a stand-alone package somewhere outside of the sdk sub-package.

Additional Context

#2501 described the introduction of the stand-alone mapping functions for use by other OpenTelemetry components.